Microlithographic illumination method and a projection lens for carrying out the method

Abstract

A microlithographic illumination method for imaging a pattern arranged in an object plane of a projection lens onto an image plane of the projection lens, under which a special means for optically correcting the optical path lengths of s-polarized and p-polarized light such that light beams of both polarizations will either traverse essentially the same optical path length between the object plane and the image plane or any existing difference in their optical path lengths will be retained, largely independently of their angles of incidence on the image plane, which will allow avoiding contrast variations due to pattern orientation when imaging finely structured patterns, is disclosed. The contrast variations may be caused by uncorrected projection lenses due to their employment of materials that exhibit stress birefringence and/or coated optical components, such as deflecting mirrors, that are used at large angles of incidence.

What is claimed is:  
     
       1. A method for imaging a pattern arranged in an object plane of a projection lens onto an image plane of the projection lens comprising: 
       illuminating the pattern with light for creating a light beam with a first light ray having a first polarization direction and a second light ray having a second polarization direction that differs from the first polarization direction;  
       transmitting the light beam through the projection lens, wherein the light rays of the light beam are incident on the optical components of the projection lens at differing angles of incidence and wherein a difference in the lengths of a first optical path traversed by the first light ray and a second optical path traversed by the second light ray occurs within at least one region of the image plane due to the optical configuration employed; and  
       compensating for the difference in optical path length by intentionally altering at least one of the first optical path and the second optical path such that the difference in the length of the optical path traversed by the first light ray and the length of the optical path traversed by the second light ray occurring within the image plane is largely independent of the angles of incidence of the light rays.  
     
     
       2. A method according to  claim 1 , wherein said compensating for the difference in the lengths of the optical paths of the light rays comprises: 
       introducing a non-rotational symmetric gradient in the difference in the length of the optical paths of the light rays, the gradient being oriented orthogonal to an optical axis of the projection lens.  
     
     
       3. A method according to  claim 2 , wherein the gradient is substantially linear, whereby a tilting of wavefronts running through the projection lens is at least partly compensated. 
     
     
       4. A method according to  claim 1 , wherein said compensating for the difference in the lengths of the optical paths of the light rays comprises: 
       introducing a substantially spherical gradient in the difference in the length of the optical paths of the light rays, the gradient being concentric with an optical axis of the projection lens.  
     
     
       5. A method according to  claim 1 , wherein the compensation for the difference in the lengths of the optical paths is introduced in a vicinity of a pupil plane of the projections lens. 
     
     
       6. A method according to  claim 1 , wherein the compensation for the difference in the lengths of the optical paths is introduced in a vicinity of a field plane of the projection lens such that a phase shift between the light rays having differing polarizations that varies over the field plane is introduced in conjunction with the compensation. 
     
     
       7. A method according to  claim 6 , wherein the compensation is introduced in a vicinity of an intermediate image. 
     
     
       8. A method according to  claim 2 , wherein the compensation for the difference in the lengths of the optical paths is introduced in a vicinity of a pupil plane of the projections lens. 
     
     
       9. A method according to  claim 2 , wherein the compensation for the difference in the lengths of the optical paths is introduced in a vicinity of a field plane of the projection lens such that a phase shift between the light rays having differing polarizations that varies over the field plane is introduced in conjunction with the compensation. 
     
     
       10. A method according to  claim 9 , wherein the compensation is introduced in a vicinity of an intermediate image. 
     
     
       11. A projection lens for a microlithographic projection system for imaging a pattern arranged in an object plane onto an image plane of the projection lens employing light that contains a first light ray having a first polarization direction and a second light ray having a second polarization direction that differs from the first polarization direction, 
       the projection lens comprising optical components that may be illuminated by transmitted light rays at varying angles of incidence,  
       wherein a difference in the length of a first optical path traversed by the first light ray and a second optical path traversed by the second light ray occurs within at least one region of the image plane due to the optical configuration of the optical components;  
       the projection lens further comprising at least one optical correction means for correcting for differences in the optical path length of the light rays by altering at least one of the first optical path and the second optical path,  
       the optical correction means being configured such that the difference in the optical path length occurring at the image plane is largely independent of the angles of incidence of the light rays on the optical components.  
     
     
       12. A projection lens according to  claim 11 , wherein the optical correction means are configured for creating a non-rotational symmetric gradient in a phase shift between the light rays having differing polarization directions, the gradient being oriented roughly orthogonal to the optical axis of the projection lens. 
     
     
       13. A projection lens according to  claim 12 , wherein the optical correction means are configured for creating a substantially linear gradient of the phase shift, whereby a tilting of wavefronts running through the projection lens is at least partly compensated. 
     
     
       14. A projection lens according to  claim 12 , wherein the optical correction means include at least one wedge-shaped retarding element fabricated from a birefringent material. 
     
     
       15. A projection lens according to  claim 12 , wherein the optical correction means include a retarding element fabricated from a birefringent material and an optical element fabricated from an optically isotropic material, wherein the optically isotropic optical element is shaped complementary to the retarding element and is aligned with respect to the the retarding element such that the retarding element and the optically isotropic optical element form a plane-parallel optical element. 
     
     
       16. A projection lens according to  claim 12 , wherein the optical correction means include at least one retarding element fabricated from a birefringent material having a gradient of the refractive index transverse to the optical axis of the projection lens. 
     
     
       17. A projection lens according to  claim 12 , wherein the optical correction means have the overall shape of a plane-parallel plate. 
     
     
       18. A projection lens according to  claim 12 , wherein the projection lens is a catadioptric projection lens having at least one concave mirror and at least one deflecting mirror inclined with respect to the optical axis and arranged between the object plane and image plane. 
     
     
       19. A projection lens according to  claim 12 , further comprising at least one pupillary surface, wherein at least one optical correction means is arranged in the vicinity of the pupillary surface. 
     
     
       20. A projection lens according to  claim 12 , comprising at least one real intermediate image situated in the vicinity of an intermediate image plane between the object plane and the image plane, wherein at least one optical correction means is arranged in the vicinity of the intermediate image plane. 
     
     
       21. A projection lens according to  claim 11 , wherein the optical correction means are configured for creating a substantially spherical gradient of the phase shift, the gradient being concentric with the optical axis of the correction means. 
     
     
       22. A projection lens according to  claim 21 , wherein the optical correction means include a retarding element fabricated from a birefringent material that is axially bounded by a planar surface on one axial end and a curved surface on the opposite axial end. 
     
     
       23. A projection lens according to  claim 11 , wherein the optical correction means include a retarding element fabricated from a birefringent material and an optical element fabricated from an optically isotropic material, wherein the optically isotropic optical element is shaped complementary to the retarding element and is aligned with respect to the the retarding element such that the retarding element and the optically isotropic optical element form a plane-parallel optical element. 
     
     
       24. A projection lens according to  claim 11 , wherein the optical correction means include at least one retarding element fabricated from a birefringent material having a gradient of the refractive index transverse to the optical axis of the projection lens. 
     
     
       25. A projection lens according to  claim 11 , wherein the optical correction means have the overall shape of a plane-parallel plate. 
     
     
       26. A projection lens according to  claim 11 , wherein the projection lens is a catadioptric projection lens having at least one concave mirror and at least one deflecting mirror inclined with respect to the optical axis and arranged between the object plane and image plane. 
     
     
       27. A projection lens according to  claim 26 , further comprising a catadioptric first section having a concave mirror and a beam-deflecting device and a dioptric second section following the first section, wherein the beam-deflecting device preferably has a first reflecting surface for deflecting radiation coming from the object plane to the concave mirror and a second reflecting surface for deflecting radiation reflected by the concave mirror toward the second section. 
     
     
       28. A projection lens according to  claim 27 , wherein the first deflecting mirror is oriented at an angle with respect to the optical axis of the projection lens, the angle deviating from 45° such that an angle through which it deflects incident radiation to the concave mirror is greater than 90°. 
     
     
       29. A projection lens according to  claim 26 , further comprising at least one pupillary surface, wherein at least one optical correction means is arranged in the vicinity of the pupillary surface. 
     
     
       30. A projection lens according to  claim 26 , comprising at least one real intermediate image situated in the vicinity of an intermediate image plane between the object plane and the image plane, wherein at least one optical correction means is arranged in the vicinity of the intermediate image plane. 
     
     
       31. A projection lens according to  claim 26 , wherein the optical correction means are configured for creating a non-rotational symmetric gradient in a phase shift between the light rays having differing polarization directions, the gradient being oriented roughly orthogonal to the optical axis of the projection lens. 
     
     
       32. A projection lens according to  claim 31 , wherein the optical correction means are configured for creating a substantially linear gradient of the phase shift, whereby a tilting of wavefronts running through the projection lens is at least partly compensated. 
     
     
       33. A projection lens according to  claim 31 , wherein the optical correction means include at least one wedge-shaped retarding element fabricated from a birefringent material. 
     
     
       34. A projection lens according to  claim 26 , wherein the optical correction means are configured for creating a substantially spherical gradient of the phase shift, the gradient being concentric with the optical axis of the correction means. 
     
     
       35. A projection lens according to  claim 34 , wherein the optical correction means include a retarding element fabricated from a birefringent material that is axially bounded by a planar surface on one axial end and a curved surface on the opposite axial end. 
     
     
       36. A projection lens according to  claim 26 , wherein the optical correction means include a retarding element fabricated from a birefringent material and an optical element fabricated from an optically isotropic material, wherein the optically isotropic optical element is shaped complementary to the retarding element and is aligned with respect to the the retarding element such that the retarding element and the optically isotropic optical element form a plane-parallel optical element. 
     
     
       37. A projection lens according to  claim 26 , wherein the optical correction means include at least one retarding element fabricated from a birefringent material having a gradient of the refractive index transverse to the optical axis of the projection lens. 
     
     
       38. A projection lens according to  claim 26 , wherein the optical correction means have the overall shape of a plane-parallel plate. 
     
     
       39. A projection lens according to  claim 11 , further comprising at least one pupillary surface, wherein at least one optical correction means is arranged in the vicinity of the pupillary surface. 
     
     
       40. A projection lens according to  claim 11 , comprising at least one real intermediate image situated in the vicinity of an intermediate image plane between the object plane and the image plane, wherein at least one optical correction means is arranged in the vicinity of the intermediate image plane. 
     
     
       41. A method for fabricating a projection lens comprising optical components and being usable for transmitting light containing a first light ray having a first polarization direction and a second light ray having a second polarization direction that differs from the first polarization direction, wherein the optical components may be illuminated by a transmitted light beam containing light rays having differing angles of incidence on the optical components and wherein a difference in the lengths of a first optical path traversed by the first light ray and a second optical path traversed by the second light ray occurs within at least one region of the image plane of the projection lens due to the optical configuration employed, wherein the method comprises: 
       configuring the projecting lens using the optical components;  
       transmitting light beams through the projection lens, wherein the light beams contain a first light ray and a second light ray that is orthogonally polarized with respect to the first light ray;  
       determining wavefronts for light rays transmitted by the projection lens, wherein a first wavefront for the first light ray and a second wavefront for the second light ray are determined in accordance with their polarizations, and the wavefronts are employed for determining differential wavefronts;  
       creating at least one optical correction means in accordance with the differential wavefronts, wherein the optical correction means are configured for compensating for the differences in the lengths of the optical paths such that the differential wavefronts will be largely independent of their angles of incidence on the optical components due to the compensation for the differences in the lengths of the optical paths when the optical correction means is installed in the projection lens; and  
       installing that optical correction means in the projection lens.  
     
     
       42. A method according to  claim 41 , wherein the configuration and illumination of the projection lens, the determinations of the wavefronts and the differential wavefronts, and the creation of at least one means of optical correction, are performed computer-assisted. 
     
     
       43. A method according to  claim 42 , wherein at least one optical correction means is fabricated based on parameters that have been determined with the assistance of a computer. 
     
     
       44. A method according to  claim 41 , wherein the differential wavefront is empirically determined on an actual, fully assembled projection lens employed for empirically determining the parameters for fabricating the means of optical correction and the means of optical correction is fabricated based on the empirically determined parameters. 
     
     
       45. An optical corrector for installation in a projection lens for a microlithographic projection system for imaging a pattern arranged in an object plane onto an image plane of the projection lens employing light that contains a first light ray having a first polarization direction and a second light ray having a second polarization direction that differs from the first polarization direction, the projection lens comprising optical components that may be illuminated by transmitted light rays at varying angles of incidence, 
       wherein a difference in the length of a first optical path traversed by the first light ray and a second optical path traversed by the second light ray occurs within at least one region of the image plane due to the optical configuration of the optical components,  
       the optical corrector being adapted for correcting for differences in the optical path length of the light rays by altering at least one of the first optical path and the second optical path,  
       the optical corrector being configured such that the difference in the optical path length occurring at the image plane is largely independent of the angles of incidence of the light rays on the optical components.  
     
     
       46. An optical corrector according to  claim 45 , wherein the optical corrector is configured for creating a substantially linear gradient of a phase shift, whereby a tilting of wavefronts running through the projection lens is at least partly compensated. 
     
     
       47. An optical corrector according to  claim 46 , comprising at least one wedge-shaped retarding element fabricated from a birefringent material. 
     
     
       48. An optical corrector according to  claim 46 , comprising a retarding element fabricated from a birefringent material and an optical element fabricated from an optically isotropic material, wherein the optically isotropic optical element is shaped complementary to the retarding element and is aligned with respect to the retarding element such that the retarding element and the optically isotropic optical element form a plane-parallel optical element. 
     
     
       49. An optical corrector according to  claim 46 , comprising at least one retarding element fabricated from a birefringent material having a gradient of the refractive index transverse to the optical axis of the optical corrector. 
     
     
       50. An optical corrector according to  claim 45 , comprising a retarding element fabricated from a birefringent material and an optical element fabricated from an optically isotropic material, wherein the optically isotropic optical element is shaped complementary to the retarding element and is aligned with respect to the retarding element such that the retarding element and the optically isotropic optical element form a plane-parallel optical element. 
     
     
       51. An optical corrector according to  claim 45 , comprising at least one retarding element fabricated from a birefringent material having a gradient of the refractive index transverse to the optical axis of the optical corrector. 
     
     
       52. A method for manufacturing semiconductor devices or other types of micro-devices comprising: 
       arranging a mask having a prescribed pattern in the object plane of a projection lens;  
       illuminating the mask using ultraviolet light;  
       using a projection lens to project an image of the pattern onto a photosensitive substrate situated in the vicinity of the image plane of the projection lens;  
       wherein the projection lens comprises optical components illuminated by transmitted light rays at varying angles of incidence,  
       wherein a difference in the length of a first optical path traversed by a first light ray having a first polarization direction and a second optical path traversed by a second light ray having a second polarization direction that differs from the first polarization direction occurs within at least one region of the image plane due to the optical configuration of the optical components;  
       wherein the projection lens further comprises at least one optical correction device that corrects differences in the optical path length of the light rays by altering at least one of the first optical path and the second optical path, and  
       wherein the optical correction device is configured to render the difference in the optical path length occurring at the image plane substantially independent of the angles of incidence of the light rays on the optical components.  
     
     
       53. A method for manufacturing semiconductor devices or other types of micro-devices comprising: 
       arranging a mask having a prescribed pattern in the object plane of a projection lens;  
       illuminating the mask using ultraviolet light; and  
       using a projection lens to project an image of the pattern onto a photosensitive substrate situated in the vicinity of the image plane of the projection lens;  
       wherein the projection lens is a catadioptric projection lens having at least one concave mirror and at least one deflecting mirror inclined with respect to the optical axis and arranged between the object plane and image plane; and  
       wherein the projection lens comprises optical components illuminated by transmitted light rays at varying angles of incidence, wherein a difference in the length of a first optical path traversed by a first light ray having a first polarization direction and a second optical path traversed by a second light ray having a second polarization direction that differs from the first polarization direction occurs within at least one region of the image plane due to the optical configuration of the optical components;  
       wherein the projection lens further comprises at least one optical correction device that corrects differences in the optical path length of the light rays by altering at least one of the first optical path and the second optical path;  
       wherein the optical correction device is configured to render the difference in the optical path length occurring at the image plane substantially independent of the angles of incidence of the light rays on the optical components; and  
       wherein the optical correction device is configured for creating a substantially linear gradient of the phase shift, whereby a tilting of wavefronts running through the projection lens is at least partly compensated.